News from the Ice Wars:An Occasional Bulletin

The two largest ice sheets on earth, Greenland and Antarctica, are losing mass faster and faster. The figure below is from Velicogna and Wahr, "Time-variable gravity observations of ice sheet mass balance: Precision and limitations of the GRACE satellite data" GEOPHYSICAL RESEARCH LETTERS, VOL. 40, 3055-3063, doi:10.1002/grl.50527, 2013

Mass loss for Greenland and 
Antarctica from 2003-2013

For reference, 360 GTonne of mass loss results in 1 mm sea level rise. Greenland lost more than that in 2012 alone.

Causes of Mass Loss

I. Marine terminated glaciers flow to the ocean and calve off icebergs. The figure below is from Rignot and Mouginot,"Ice flow in Greenland for the International Polar Year 2008-2009," GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L11501, doi:10.1029/2012GL051634, 2012

Ice velocity map of Greenland

Note Jacobshawn in the west and the great double snouted North East Greenland Ice Stream.

II. Warmer ocean waters melt ice shelves at their bases. I show Fig 1 from Rignot et al.,"Ice Shelf Melting Around Antarctica," Science Express, 13th June 2013, doi:10.1126/science.1235798

Basal Melt of Antarctic 
Ice Shelves
Note the sectors in the Amundsen sea, and to a lesser extent, near Totten. These ice shelves hold back inland ice, and as the disappear, ice flow inland accelerates. As Pritchard et al.,"Antarctic ice-sheet loss driven by basal melting of ice shelves,"Nature, 16 April 2012, v484, pp502 et seq., doi:10.1038/nature10968 put it: "Every case of ocean-driven ice-shelf thinning that we identify is linked with previously documented dynamic thinning of grounded, fast-flowing tributary glaciers ..." Indeed, as they show in Fig. 2 below, the greatest inland ice mass loss is in regions fringed by rapidly melting ice shelves.
Antarctic inland mass loss 
and rates of ice shelf melt

There is a key feature in the bathymetry depicted above: the rapidly thinning shelves are adjacent to deep troughs in the ocean bottom, that allow incursions of relatively warm Circumpolar Deep Water which cause basal melt.

III. Darker snow surfaces absorb more sunlight and melt quicker. This effect is visible in Greenland. Box et al.,"Greenland ice sheet albedo feedback: thermodynamics and atmospheric drivers," The Cryosphere, v6, pp821 et seq., doi:10.5194/tc-6-821-2012 state: " Thus, intraseasonal summer albedo variability exceeds 0.50 over parts of the ice sheet where a snow layer ablates by mid-summer, exposing an impurity-rich ice surface ..." I reproduce Fig. 6 below

Variation in Greenland albedo from 
2000 to 2011

Note the steady decline. Prof. Box is now investigating soot from North American wildfires as a factor in the darkening of Greenland. Some more information on Greenland albedo and the extraordinary melt in 2012 may be found here.

These factors affect Greenland and Antarctica to different extents. In Greenland surface melt has started to dominate over the last few years, as Enderlin et al. show in Enderlin(2014), "An improved mass budget for the Greenland ice sheet," Geophys. Res. Lett., 41, 2014, doi:10.1002/2013GL059010

Contributions to mass
waste in Greenland from glacier discharge and Surface Mass Balance

whereas in Antarctica basal melting is more important. In both cases, major ice streams toward the ocean have accelerated.

The result is an addition of water to the world oceans, which raises sea level. The rate of increase of mass waste from the two major ice sheets is therefore a question of great importance, for while we might easily deal with a small rate of sea level rise, we would face more and more difficulty as that rate increased.

We may look to the past for some answers. At the end of the last ice age, the ice sheets retreated and sea level rose. One of these episodes of ice melt accelated SLR is called Melt Water Pulse 1A (MWP1A), which occurred about fourteen millennia ago, featured a twenty meter (sixty foot) rise in a total period of two to five centuries. Even at the slower estimate, this is a sea level rise of 1 meter (3 feet) every twenty five years, or more than a foot a decade. The faster estimate is one meter (three feet) a decade. Both of these rates of SLR would drown coasts worldwide with horrific consequences.The population of low elevation (under 10 meters) coastal zones is around 700 million souls, all of whom would be drowned or displaced within a century or two. The following graph is from Mondal and Tatem( 2012), Uncertainties in Measuring Populations Potentially Impacted by Sea Level Rise and Coastal Flooding. PLoS ONE 7(10):e48191. doi:10.1371/journal.pone.0048191

Population estimates by 
continent living below 10 meters elevation using two different models

Even a meter of sea level rise would affect about 180 million people. Here is a graph from Nichols et al., "Sea-level rise and its possible impacts given a 'beyond 4°C world' in the twenty-first century", Phil. Trans. R. Soc. A 2011 369, doi: 10.1098/rsta.2010.0291

Populations affected for 0.5 m 
and 2.0m sea level rise by country

Current projections of the fate of these two icesheets are uncertain. Projections using linear, semi-empirical models project a range of 0.5 to 2 meters by 2100. These do not and cannot take into account nonlinear processes destabilizing the ice sheets.

Some more posts on Greenland and sea level rise are linked from here